1. Field of the Invention
The present invention relates measuring and testing devices, and particularly to a telescopic ball bar gauge for testing and calibrating small scale numerical controlled machine tools, coordinate measuring machines and the like.
2. Description of the Related Art
FIG. 2 illustrates a typical prior art coordinate measuring machine (CMM) 100. A coordinate measurement machine is typically used to map or measure the shape and dimensions of an article. For this purpose, such a CMM often includes a measuring envelope or chamber within which the article to be measured is positioned, such as upon a suitable support or base. A probe is then moved within the measuring envelope to contact either preselected points or randomly selected points upon the article. The movements of the probe are recorded and may be translated into three-dimensional readings, as for example, into X-, Y-, and Z-axis readings, or into other useful data. Thus, when the probe contacts a point on the surface of the article, a signal is produced that is converted into coordinate measurements or other data through a suitably programmed computer.
The measurements produced by the coordinate measuring machine probe contacting the article may be used for a variety of purposes, including producing drawings of the article, determining surface shapes or contours, assisting in designing the article, determining dimensions and volumes, etc. The sizes and construction of coordinate measurement machines may vary, but typically they are of a substantial size in order to measure substantial size items. For example, a coordinate measurement machine may be used to measure an automotive engine block, or it may be used to measure an automobile body or similarly large articles.
Because accurate measurements are usually required, the accuracy of the coordinate measuring machine itself must be periodically tested. For that purpose, a suitable gauge is needed for producing test data generated by test movements and contacts of the probe. That test data can be used to determine equipment inaccuracies and needed compensations for any such inaccuracies.
The probe in a coordinate measurement machine is typically mounted upon an arm connected to a movable support system by which the probe may be moved three-dimensionally within the envelope or chamber or volumetric area within which the measuring is performed. Thus, inaccuracies may arise because of varying tolerances or dimensional inaccuracies of the parts of the probe support system. Such inaccuracies may vary at different places or probe positions within the envelope. It is desirable to provide a gauge system that can be used to determine the accuracy of the probe-generated measurements between spaced-apart points located at numerous places within the envelope. This is particularly necessary because the magnitude of measurement inaccuracies may change materially at the outer areas of the measuring envelope as compared with central locations of the envelope due to greater movements of the probe support system at the outer areas.
FIG. 2 schematically illustrates a coordinate measuring machine 100 that is used to measure the shapes and dimensions of surfaces of an article, such as an engine block 111, or a larger article, such as an automobile body or body part, or smaller articles. The machine 100 includes a floor or platform 112 upon which a base 113 is located for supporting a support plate 114 upon which the article is positioned.
The article is repeatedly contacted by a probe 115, which is mounted upon an arm 116 that moves towards and away from the article. The arm 116 is carried by an arm support block 117, which is slidably mounted upon a vertical post 118 for upwards and downwards movement, as indicated by the double-headed arrow adjacent the end of the block 117. The post 118 is carried by a post support base 119, which is slidably positioned within a post base guide channel 120 for reciprocal movement, as indicated by the double-headed arrow at the guide channel 120.
The construction of coordinate measuring machines may vary considerably, but the general construction and operation of a typical CMM involves a probe, which is mounted for movement relative to the article to be measured so that the probe can contact selected portions of the article and the movements and locations of the probe can be detected. The article itself is contained within a measuring “envelope” or chamber; i.e., the area surrounding the article. In practice, the chamber or envelope 121 may be formed by a walled, room-like area. Further, although the probe 115 is shown in FIG. 2 as being mounted for three-dimensional movement by the inward and outward movement of the arm 116 carrying the probe 115, the up and down movement of the block 117 carrying the arm 116 and the backwards and forward movement of the post 118 to which the arm 116 is connected, the probe 115 may also be mounted upon a more universally movable support, such as a conventional X-Y or X-Y-Z movable stage, as is conventionally known.
The movement of the probe 115 in contacting various parts of the article 111 may be controlled either manually or by a suitable mechanism located remotely from the probe 115. When the probe 115 contacts the article 111, a signal is transmitted through a wiring system 122 to a computer 123, which provides data that is read out through a printer or screen or the like 124, or is otherwise used in some other data-responsive equipment. It should be understood that the computer 123 and the display or interface 124 are schematically illustrated to show the general relationship of the relevant components.
In using a coordinate measuring machine, such as that illustrated in FIG. 1, close accuracy of the readings is required. However, the nature of the equipment, including the tolerances required in making the parts of the equipment, as well as the looseness of the parts resulting from wear, affects the accuracy of the readings. Thus, the generated measurement readings may be more or less accurate at various places within the measuring envelope or chamber. For example, locations that are central within the chamber are likely to be more accurate than places located along the edges or outer fringes of the measuring envelope, where more probe movement is needed. These discrepancies or inaccuracies in measurements can substantially affect the data produced and later uses of that data. Thus, it is important to know, by fairly regularly taking test measurements, the varying inaccuracies of the equipment, including at different places within the measuring envelope, so that these inaccuracies can be considered when utilizing the data produced by the machine.
In order to test the accuracy of a coordinate measuring machine, a ball bar gauge is typically used. A conventional ball bar gauge provides a pair of movable test measuring points that are spaced apart a known distance so that the points may be contacted by the probe of a coordinate measuring machine to generate test data. Typically, the ball bar gauge is in the form of a movable device that can be periodically placed within a measuring envelope of the CMM, and can be moved into a variety of test measuring positions, and then removed from the CMM.
FIG. 3 illustrates a typical prior art magnetic ball bar gauge 200 for evaluating the performance of coordinate measuring machines, such as the CMM 100 of FIG. 2. In use, a fixed magnetic socket 210 is mounted on a coordinate measuring machine workpiece supporting surface (such as the support plate 114 of FIG. 2), and a tapered insert 209 of the free socket 208 is mounted in the coordinate measuring machine probe holder (either by attachment of a socket to the probe 115, or by removal of the probe 115 and attachment to the arm 116). Tooling balls 206 and 207 are then placed in the sockets 208 and 210. The tooling balls 206 and 207 are conventional spherical tooling balls that have tolerances well below the tolerances intended to be measured. Ball bar gauge 200 is then placed on balls 206 and 207, where it is held by magnetic attraction from the magnets in the chucks 202 and 203.
A rod 201 is attached to the two magnetic chucks 202 and 203 at either end. Each of the chucks 202 and 203 has tapered ends 204 and 205, respectively, which provide clearance between the chucks 202 and 203, and the sockets 208 and 210 when the bar 200 is moved into different positions in accordance with standard measuring procedures.
FIG. 2 illustrates a conventional CMM for providing measurements of a large-scale object, such as the exemplary engine block 111. A ball bar gauge for testing the accuracy of such a CMM, such as the gauge 200 of FIG. 3, which largely relies on mechanical measurement, would be dimensioned and configured on an equivalent scale. However, a conventional ball bar gauge, such as the gauge 200, would not be able to provide accurate readings for microscale CMMs, which require a far greater level of accuracy, as well as a far smaller size.
Ball bars and their use in calibrating machines are well known in the art. U.S. Pat. No. 4,435,905, to Bryan, illustrates a typical exemplary ball bar system, which is herein incorporated by reference in its entirety. This system consists of a telescopic rod having a ball at each end, and which can be positioned between a socket carried by the spindle of a machine, and a socket mounted on the machine table. The spindle is driven around in a circle about the center of the table-mounted socket, for example in a horizontal, x,y plane, and measurements are made of any change in length of the ball bar by means of a transducer in, or on, the telescopic rod.
The most accurate calibration of the errors in the spindle movement around the circle are obtained when the axis of the ball bar lies in a plane which includes the centre of the ball in the socket, because then errors in the movement of the spindle along its axis do not affect the length of the ball bar. Thus, the results of the test reflect only errors of movement of the spindle in the x,y plane.
It is possible to make some measurement of errors in the spindle movement in the vertical x,z or y,z planes through the centre of the ball in the table-mounted socket, but with the apparatus set up as described above, the spindle cannot move through more than 180° in these planes without coming into contact with the ball bar or the socket support.
A circle through 360° can be made by the spindle in a vertical plane alongside the table-mounted socket, and sufficiently offset from the socket that such contact of the spindle with the socket or the ball bar is avoided. However, with this set up the ball bar will lie at an angle to the vertical plane, and its length can be varied by movements of the spindle in the horizontal plane which can give rise to errors in the calibration of the vertical plane.
Alternatively, the socket could be repositioned so that it is possible to enable the spindle to perform a 360° circle centered on the ball in the socket and with the ball bar lying in a vertical plane through the centre of the ball in the socket. This, however, has disadvantages, particularly when acting in all three planes, where three separate operations may be needed. This is time-consuming in itself and, further, three different positions may be required for the socket, thus causing calibrations which are not performed about the same center and, thusly, are not easily correlated.
It would be desirable to provide an apparatus for calibrating all three planes of a machine and which allows a ball bar in a machine spindle to be driven around 360° separately in each of three orthogonal planes of the machine with the ball bar centered on a single fixed position, and with the axis of the ball bar lying in the respective calibration plane each time.
Additionally, presently available ball bars can neither test nor calibrate the new generations of micro-machine tools and micro-CMMs because of their relatively large dimensions and bulky prototypes. It would be desirable to provide a telescopic ball bar which could be used to easily test and calibrate such machines, in particular, and also be used with standard scale machines. Thus, a telescopic ball bar gauge solving the aforementioned problems is desired.